The present disclosure relates to a coordinate measuring device. One set of coordinate measurement devices belongs to a class of instruments that measure the three-dimensional (3D) coordinates of a point by sending a laser beam to the point, where it is intercepted by a retroreflector target. The instrument finds the coordinates of the point by measuring the distance and the two angles to the target. The distance is measured with a distance-measuring device such as an absolute distance meter (ADM) or an interferometer. The angles are measured with an angle-measuring device such as an angular encoder. A gimbaled beam-steering mechanism within the instrument directs the laser beam to the point of interest. Examples of such a device include a laser tracker, a total station, and a time-of-flight (TOF) scanner.
Ordinarily the laser tracker sends a laser beam to a retroreflector target. A common type of retroreflector target is the spherically mounted retroreflector (SMR), which comprises a cube-corner retroreflector embedded within a metal sphere. The cube-corner retroreflector comprises three mutually perpendicular mirrors. The apex of the cube corner, which is the common point of intersection of the three mirrors, is located at the center of the sphere. It is common practice to place the spherical surface of the SMR in contact with an object under test and then move the SMR over the surface being measured. Because of this placement of the cube corner within the sphere, the perpendicular distance from the apex of the cube corner to the surface of the object under test remains constant despite rotation of the SMR. Consequently, the 3D coordinates of a surface can be found by having a tracker follow the 3D coordinates of an SMR moved over the surface. It is possible to place a glass window on the top of the SMR to prevent dust or dirt from contaminating the glass surfaces.
A gimbal mechanism within the laser tracker may be used to direct a laser beam from the tracker to the SMR. Part of the light retroreflected by the SMR enters the laser tracker and passes onto a position detector. The position of the light that hits the position detector is used by a tracker control system to adjust the rotation angles of the mechanical azimuth and zenith axes of the laser tracker to keep the laser beam centered on the SMR. In this way, the tracker is able to follow (track) the SMR.
Angular encoders attached to the mechanical azimuth and zenith axes of the tracker may measure the angle of rotation about the azimuth and zenith axes of the laser beam (with respect to the tracker frame of reference). The one distance measurement and two angle measurements performed by the laser tracker are sufficient to completely specify the three-dimensional location of the SMR.
As mentioned previously, two types of distance meters may be found in laser trackers: interferometers and absolute distance meters (ADMs). In the laser tracker, an interferometer (if present) may determine the distance from a starting point to a finishing point by counting the number of increments of known length (usually the half-wavelength of the laser light) that pass as a retroreflector target is moved between the two points. If the beam is broken during the measurement, the number of counts cannot be accurately known, causing the distance information to be lost. By comparison, the ADM in a laser tracker determines the absolute distance to a retroreflector target without regard to beam breaks, which also allows switching between targets. Because of this, the ADM is said to be capable of “point-and-shoot” measurement. Initially, absolute distance meters were only able to measure stationary targets and for this reason were always used together with an interferometer. However, some modern absolute distance meters can make rapid measurements, thereby eliminating the need for an interferometer.
Some laser trackers include one or more cameras. A camera axis may be coaxial with the measurement beam or offset from the measurement beam by a fixed distance or angle. A camera may be used to provide a wide field of view to locate retroreflectors. A modulated light source placed near the camera optical axis may illuminate retroreflectors, thereby making them easier to identify. In this case, the retroreflectors flash in phase with the illumination, whereas background objects do not. One application for such a camera is to detect multiple retroreflectors in the field of view and measure each in an automated sequence.
Some laser trackers have the ability to measure with six degrees of freedom (DOF), which may include three coordinates, such as x, y, and z, and three rotations, such as pitch, roll, and yaw. Several systems based on laser trackers are available or have been proposed for measuring six degrees of freedom.
Laser scanners determine the 3D coordinates of points on an object surface by projecting a beam of light directly onto the surface and then collecting and analyzing the reflected light. Laser scanners are typically used for scanning closed or open spaces such as interior areas of buildings, industrial installations and tunnels. Laser scanners are used for many purposes, including industrial applications and accident reconstruction applications. A laser scanner can be used to optically scan and measure objects in a volume around the scanner through the acquisition of surface points representing objects within the volume.
Some contemporary laser scanners may also include a camera mounted on or integrated into the laser scanner for gathering camera digital images of the environment and for presenting the camera digital images to an operator. By viewing the camera images, the operator can determine the extent of the measured volume and adjust the settings of the laser scanner to measure over a larger or smaller region of space. In addition, the camera digital images may be transmitted to a processor to add color to the scanner image.
The acquisition of three-dimensional coordinates of surface points by laser scanners may result in a large volume of data involving millions of surface points. Many of these surface points may not be needed in order to adequately represent objects or surfaces within the scanned volume. Some extraneous data may be removed during postprocessing.
It is often the case that it is useful to characterize in three dimensions the environment surrounding the 3D measuring instrument. Ordinarily such information must be provided by an operator. Accordingly, while existing 3D measuring instruments are suitable for their intended purposes the need for improvement remains.